Electrophotographic printing devices, typically referred to as laser printers, are designed to create an image by printing a series of dots on a print medium, typically paper. This image is created by a highly focused light source which is scanned at a specific rate across a charged surface of photosensitive material, typically referred to as the drum. This light source is modulated such that some areas are exposed and some are not, creating a predetermined pattern on the photosensitive material. The areas sensitized by the light source cause the material to bear a charge pattern corresponding to the desired image to be printed. The final printed material is created by the attraction of toner particles to the sensitized areas of photosensitive material and then transferring this toner to the print media.
The majority of electrophotographic print engines are developed by a few large manufacturers, e.g. Canon, Fuji-Xerox, Lexmark, Minolta, and Toshiba, and require the addition of a graphics controller which describes in an electronic form the page to be printed on the marking engine. Being that the largest customer desire is to produce the highest quality positive output, these devices are each individually tuned at the manufacturing factory to produce the best possible positive paper output. Solid state lasers are used by these vendors and each produces a slightly different laser intensity. This intensity is measured during the manufacturing process and a mechanical potentiometer is adjusted to bring the desired laser intensity to within manufacturing specifications. This mechanical potentiometer is then glued in order to secure the desired value for the life of the product.
Many users desire negative image output. That is, they want the printed output to be reversed, white areas are black and black areas are white. Typically the negative output is printed on a clear polyester film. Negatives are used in applications where ultraviolet light is applied to the negative output and penetrates the white (blank) areas to harden or weaken an emulsion such as for creating metal plate masters for an offset press, silk-screen masters and masters for rubber stamp creation. However, when a standard electrophotographic printer is used to create the negative output, the laser power, which is tuned to create positive output, is too strong and erases the white areas of the output (FIG. 6).
The laser in an electrophotographic device in theory only emits light at the precise area of the electrophotographic drum defined by the resolution of the printing device, typically 1/600 of an inch. In reality, in order to produce solid black areas and guarantee toner adhesion of the smallest light sensitized areas of the drum, the laser beam is actually about twice the width of the stated resolution of the print engine. The beam overlap created by the wider laser enhances positive output but causes image loss by excess toner attraction and therefore image loss on negative output. By decreasing the intensity of the solid state laser when creating negative output, the area of the drum sensitized by the laser is decreased and image integrity is maintained. As a byproduct of this decreased laser power, the black areas of the output grayed. These areas can be then blacked by an external process in order to block ultraviolet light, necessary when using negatives in the desired applications.
Many OEM print engines are available. Printer controller developers integrate their controllers into these OEM print engines and strive to differentiate their printers to enhance their particular market share. Typically, features are controller dependent functions such as emulations, fonts, and processing performance. In electrophotographic printers first a RAM based image of the page to be printed is created on the graphic controller at the resolution of the marking engine. This graphic controller then communicates with another controller which controls the mechanical aspects of the marking engine. This mechanical controller, typically called the DC controller, among other tasks, has primary control of two key elements of the engine, the main motor and laser scanner motor. The main motor is responsible for all media movement of the marking engine. The laser scanner is responsible for spinning the rotating mirror used to reflect the laser beam and therefore scan the laser beam across the moving photosensitive drum.
When the graphic controller communicates to the DC controller to start the printing process, the DC controller then starts the main and laser scanner motors. Paper movement now begins and is controlled by the main motor. This digital image of the page is then transferred to the light sensitive drum. The rate at which this transfer takes place is proportional to the rated speed of the marking engine. The drum rotates through a toner bin and toner is attracted to the light sensitized area of the drum. Toner is transferred to paper (or other media as appropriate) when toner is attracted away from the drum and to a highly charged roller located behind the media and intercepted by the media. The media is then heated by a fusing roller and toner is melted into the paper.